In a carriage printer, such as an inkjet carriage printer, a printhead is mounted in a carriage that is moved back and forth across the region of printing. To print an image on a sheet of paper or other recording medium (sometimes generically referred to as paper herein), the recording medium is advanced a given distance along a recording medium advance direction and then momentarily stopped. While the recording medium is stopped and supported on a platen, the printhead carriage is moved along a carriage scan path. The carriage scan path extends in a direction that is substantially perpendicular to the recording medium advance direction. As it travels along the carriage scan path, controllable marking elements in the printhead record marks on the recording medium—for example by ejecting drops from an inkjet printhead. After the carriage has printed a swath of the image while traversing the recording medium, the recording medium is advanced, the carriage direction of motion is reversed, and marking repeated so that the image is formed swath by swath.
In order to produce high quality images, it is helpful to accurately locate the leading edge of the recording medium as it is advanced toward the carriage scan path. Accurate location of the leading edge permits more precise coordination of media handling as the recording medium enters the carriage scan path and can be used for timing the start of printing and for registration of image content relative to that edge to close tolerances.
Conventional solutions for leading edge detection include the use of pivoting mechanical fingers that are located at a suitable position along the media advance path and are caused to pivot upon contact with the leading edge as the medium is advanced. The movement of these devices is typically detected by a separate optical sensor that responds when a portion of the pivoting element interrupts a light path or, alternately, is moved out from a light path or moves another component with respect to a sensed light path. One example of this type of mechanism is given in U.S. Pat. No. 6,523,925 entitled “Media Leading Edge Sensor” to Driggers. Conventional solutions of this type work, but have a number of inherent shortcomings. Pivoting members can collect dust and dirt, sticking in position instead of responding as intended to the moving receiver edge. Space and components for a separate optical path must be provided, typically beneath the platen over which the receiver travels, with its own light source and sensor and associated power and signal wiring.
Competitive pressures drive the need to provide high quality printing at lower cost, as well as the need to design printing apparatus with reduced dimensions and footprint. There is a recognized need to reduce the parts count and complexity of these systems without compromising image quality and performance.